1. Field of the Invention
The present invention relates to an optical data storage medium comprising two disks laminated to each other.
2. Description of the Related Art
In recent years, DVD-Rs (additionally recordable digital video disk or additionally recordable digital versatile disk) comprising two disks laminated to each other have been developed as an optical data storage medium capable of storing a large amount of data. Among the types of DVD-Rs, there are a single-side type having a recording layer on only one side and a double-side type having recording layers on both sides. Presently, the single-side type constitutes a main stream.
In the case of the single-side type DVD-R, constituent disks may vary; for example, one disk may be provided with a reflective layer or a recording layer, while another disk may lack such a layer. Moreover, these disks may each have a reflective layer according to a different specification. On the other hand, in the case of the double-side type DVD-R, specifications of the disks are often different. For example, a print is made on the inner peripheral portion of one disk, while no such print is made on another disk. As a result, the above-mentioned difference often causes troubles. For example, already recorded disks are laminated to each other, or alternatively unrecorded disks are laminated to each other.
As a conventional practice in the case of the single-side type DVD-R, sometimes a substrate on a read-out side is colored in order to isolate the recording layer from light having wavelengths other than a data reproducing wavelength so that the storage stability of an optical date storage medium is enhanced. The effect of coloring the substrate is particularly remarkable if the recording layer contains a coloring agent.
As for the coloring of the substrate, there are two methods. A first method comprises blending a prescribed amount of a master batch, which is prepared in advance by blending a dye or a pigment at a high concentration into pellets, into noncolored pellets. A second method comprises directly blending a coloring agent, such as a dye, a pigment, or the like, into a resin. A substrate having a desired color and a desired concentration of the coloring agent can be obtained by any of these methods if the proportions of the materials are properly controlled.
The blending of a master batch or a coloring agent into noncolored pellets or a resin, however, brings about a change in, for example, coefficient of moisture absorption to an extent that mechanical properties, such as warping, plane deflection, and others, are degraded. In the case of DVD-R, it has been found that the degradation becomes more significant as the color difference between the two substrates becomes greater.
In view of the above-mentioned facts associated with the prior art, the present invention was made. A first object of the present invention is to provide an optical data storage medium comprised of disks for lamination and having an easily distinguishable front and back.
A second object of the present invention is to provide a single-side type DVD-R having excellent mechanical properties.
The present invention provides an optical data storage medium prepared by laminating face-to-face two substrates, at least one of which has a recording layer provided thereon, such that the recording layer faces inwardly, wherein the color difference between the two substrates is 10 or more.
Further, the present invention provides an optical data storage medium prepared by laminating face-to-face two substrates, at least one of which has a recording layer provided thereon, such that the recording layer faces inwardly, wherein optically recognizable different patterns are provided on the two substrates.
As described above, if at least one of difference in color and difference in pattern is present between the two substrates, the front and the back of a disk can be easily recognized during manufacturing processes. Further, it is possible to provide a new design in order to add beauty to the disk or to allow the users to easily recognize the type of the media by color. Furthermore, it is also possible to reduce detection errors of a sensor in detecting the presence or absence of the substrate. If the color difference xcex94E between the substrates is 10 or more, the front and the back of a disk can be easily recognized by a simple color sensor. Accordingly, the present invention is effective particularly in the case of a double-side type DVD in which both of the substrates are often colored and which has a recording layer containing a coloring agent.
Furthermore, the present invention provides a single-side type optical data storage medium prepared by laminating face-to-face a disk comprising a first substrate having thereon a recording layer containing a coloring agent and a disk comprising a second substrate, wherein the ratio between the optical absorption of the first substrate at the wavelength of maximum absorption in a visible light region and the optical absorption of the second substrate at the wavelength of maximum absorption in the visible light region is set such that the smaller optical absorption is 50% or more of the larger optical absorption.
Furthermore, the present invention provides a single-side type optical data storage medium prepared by laminating face-to-face a disk comprising a first substrate having thereon a recording layer containing a coloring agent and a disk comprising a second substrate, wherein the ratio between the average optical absorption of the first substrate at wavelengths ranging from 400 to 700 nm and the average optical absorption of the second substrate at wavelengths ranging from 400 to 700 nm, is set such that the smaller average optical absorption is 50% or more of the larger average optical absorption.
The fact that the values of optical absorption of the two substrates are close to each other means that the colors or the contained percentages of the coloring agents contained in the respective substrates are close to each other. If the colors or the contained percentages of the coloring agents contained in the respective substrates become closer to each other, the difference in the amounts of moisture absorption or in mechanical properties due to the difference in colors or the difference in the contained percentages of the coloring agents becomes smaller. As a result, the mechanical properties, such as warping, plane deflection, and others, after lamination are prevented from degradation.
Further, when an ultraviolet light-curable adhesive is used for laminating the two disks, a high-power lamp such as a metal halide lamp or a xenon lamp is used for the curing of the adhesive. Since the light emitted from these lamps includes not only ultraviolet rays but also rays in a visible light region, the light in the visible light region is absorbed into the substrate and, as a result, the temperature of the substrate is raised if the substrate irradiated with the light is a colored substrate. However, since the first substrate and the second substrate have the same color in the present invention, the amounts of the light absorption of the two substrates are the same. Therefore, degradation of the mechanical properties after the lamination is not aggravated.